Further Investigations of ODDSIMS, a New Method of Dating Hydrated Obsidian

在世界各地的许多考古遗址中，最常见的文物之一是由一种叫做黑曜石的火山玻璃制成的工具、装饰品和武器。这种多用途的材料因其断裂性能、光泽的外观和锋利的刃口而受到古代和史前人民的高度重视。当适当地破碎时，原始黑曜岩块可以“剥落”成各种工具形式。对考古学家来说，黑曜石的另一个重要特性是，新剥落的黑曜石工具的新鲜表面与大气和地面中的水相互作用，产生“水化边缘”，其厚度随时间而变化。1960年，美国地质勘探局的地质学家欧文·弗里德曼（Irving Friedman）发明了一种方法，通过测量黑曜石周围薄水化边缘的宽度来确定黑曜石文物的年代。002到。01 mm厚)，用光学显微镜观察。弗里德曼博士将他的技术命名为“黑曜石水合年代测定法”，简称OHD。不幸的是，尽管这种方法有望产生廉价而准确的数据，但它失败的次数远远多于成功的次数。1998年，美国国家科学基金会拨款给田纳西大学和橡树岭国家实验室的一组研究人员，他们使用二次离子质谱法（SIMS）对OHD进行了初步的重新评估，精确地测量了水合边缘中氢的浓度与地表深度的关系。这项技术不仅提供了相互作用边缘宽度的精确测量，而且还提供了玻璃-水化学的数据。在美国国家科学基金会的三年支持下，该团队对黑曜石进行了进一步的研究，这些黑曜石是在受控的实验室条件下实验水化的，也是独立确定年代的考古标本（主要来自中美洲）。作为实验室工作的一部分，该团队开发了一种新型的反应容器系统，用于在实验室中对新鲜样品进行水化，目前正在专利审查中。到目前为止，该团队已经进行了许多重要的观察和发现。首先，他们证明了光学测量本身是不精确的，不能用于准确的年代测定。其次，他们证明了传统的黑曜石测年所用的速率方程是不正确的，因为水化速率不是随时间恒定的，而是随年龄变化的。第三，他们注意到玻璃只吸收氢，而不是像以前认为的那样吸收整个水分子，这表明氢化过程的机制比以前假设的要复杂得多，需要对玻璃-水相互作用理论进行重大的重新评估。第四，研究小组能够证明，利用氢浓度剖面的形状和深度，可以精确地确定墨西哥Chalco遗址的一系列文物的年代。然而，这种方法需要至少一个来自任何特定地点的独立日期的人工制品进行校准。第五，SIMS分析表明，边缘的氢同位素比值可能保留了水化平均温度的信息，这是控制水化速率的关键因素。如果得到证实，这将提供一种限制水合温度的方法，并使黑曜石定年成为一种完全独立的方法。最后，这项研究的结果表明，核废料玻璃形式的长期耐久性应该重新评估，因为以前的分解模型可能是基于有缺陷的假设。该研究小组已获得额外三年的支持，以继续这项令人兴奋和重要的研究。该项目未来三年的目标包括：1)了解氢化区的相互作用化学和结构；2)完善水化模型，使其成为一种完全独立、可靠的测年方法；3)确定是否可以从黑曜石伪影中提取温度数据；4)在不同的考古遗址测试这种方法。为了实现这些目标，该团队将扩大他们对旧世界黑曜石（来自希腊的米利亚黑曜石）的研究，并已经与希腊考古学家团队（爱琴海大学）建立了联系，以获取必要的样本。这使得该项目在真正的国际范围内，不仅受益于美国科学家在多个学科（如考古学，地质学，化学，物理学，材料科学），但全世界的科学家。

One of the most common artifacts at many archaeological sites across the world are tools, ornaments and weapons made from a volcanic glass called obsidian. This versatile material was highly prized by ancient and prehistoric peoples due to its fracturing properties, its lusterous appearance and its sharp cutting edge. When properly fractured, raw obsidian blocks can be "flaked" into a variety of tool forms. An additional property of obsidian that makes it valuable to archaeologists is the fact that the fresh surfaces of a newly flaked obsidian tool interact with water in the atmosphere and ground, producing a "hydration rim", the thickness of which is dependent on time. In 1960, a USGS geologist, Irving Friedman, developed a method of dating obsidian artifacts by measuring the width of the thin hydrated rims (around .002 to .01 mm thick) with an optical microscope. Dr. Friedman named his technique "obsidian hydration dating" or OHD. Unfortunately, despite the promise of the method to produce inexpensive and accurate dates, it has failed far more often than it has succeeded.In 1998, NSF awarded a grant to a team of researchers at the University of Tennessee and Oak Ridge National Laboratory who had done a preliminary reassessment of OHD using secondary ion mass spectrometry (SIMS) to precisely measure the concentration of hydrogen in the hydrated rim as a function of depth from the surface. This technique provides not only precise measurement of the width of the interaction rim, but data on the glass-water chemistry as well. During the three years of NSF support, the team carried out further research on both obsidian that was experimentally hydrated under controlled laboratory conditions and independently dated archaeological specimens (mainly from Mesoamerica). As part of the laboratory work, the team developed a new type of reaction vessel system for performing in-laboratory hydration of fresh samples, and is currently under patent review. Thus far, the team's has made a number of crucial observations and discoveries. First, they demonstrated that optical measurement is inherently imprecise and cannot be used for accurate dating. Second, they showed that the rate equations used in traditional obsidian dating are incorrect, as the rate of hydration is not constant with time, but rather varies with age. Third, they noted that the glass is absorbing only hydrogen and not the whole water molecule as previously thought, indicating that the mechanics of the hydrogenation process are far more complex than previously assumed, requiring significant re-evaluation of glass-water interaction theory. Fourth, the team was able to demonstrate that using the shape and depth of the hydrogen concentration profile, a series of artifacts from the Chalco, Mexico site could be precisely and accurately dated. However, this method requires at least one independently dated artifact from any particular site for calibration. Fifth, SIMS analysis suggests that the hydrogen isotope ratios of the rim may preserve information on the average temperature of hydration, which is a crucial factor controlling hydration rates. If confirmed, this would provide a means of constraining hydration temperature, and would allow obsidian dating to be a completely independent method. Finally, the results of this study suggest that the long-term durability of nuclear waste glass forms should be re-evaluated, as previous breakdown models may be based on flawed assumptions. The research team has been awarded an additional three years support to continue this exciting and important research. Objectives of the project for the next three years include: 1) to understand the interaction chemistry and structure of the hydrogenation zone; 2) refine the hydration model to make it a fully independent and robust dating method; 3) determine if temperature data can be extracted from obsidian artifacts; and 4) test the method at various archaeological sites. To accomplish these goals, the team will expand their research to Old World obsidian (Melian obsidian from Greece) and have already established ties with a team of Greek archaeologists (University of the Aegean) to obtain the necessary samples. This makes this project truly international in scope benefiting not only U.S. scientists in multiple disciplines (e.g. archaeology, geology, chemistry, physics, materials science), but scientists throughout the world.